"Have you ever heard of the Swiss cheese model? In order for a catastrophic event, such as a plane crash, to occur, a sequence of events precedes it. Think of these separate factors as slices of Swiss cheese lined up one behind the other. If any one of the holes in them doesn't align with the others, the series of events is changed or curtailed, and a catastrophe is prevented." "But if all the holes line up ---" "The door is open for disaster." --